Many smooth muscles maintain considerable tension, and resist stretch for long periods of time without using large amounts of energy. "Catch" in Mytilus may represent an extreme evolutionary specialization of a tension-sustaining mechanism common to many muscles, both smooth and striated. Serotonin and dopamine are neuroactive substances found in mammalian brain. The Mytilus nerve-muscle preparation is one of very few where the mechanism(s) of action of serotonin and dopamine can be studied at the cellular level and where these substances are probably released by nerves controlling relaxation. The problems of catch and serotonin and dopamine action will be approached by the following methods: 1) Completion of present biophysical studies of the action of excitatory and relaxing mediators on the muscle membrane by applying voltage-clamp methods. 2) Recording of membrane potential during neural stimulation, exploiting new methods for physiological and pharmacological separation of excitatory and relaxing nerves. 3) Analysis of Ca ions activation and Ca ions controlled changes in stretch resistance in highly permeable and chemically-"skinned" fibers. 4) Tracing of serotonin and dopamine- containing nerves from pedal ganglion to presumed endings on muscle, correlating this with known selective effects of reserpine, p-chlorophenyllalnine, guanethidine and 6-hydroxydopamine on relaxing nerve function. 5) Developing methods for isolation of single fibers or groups of fibers in tissue culture, and methods for correlation of total ion content (Ca ions in particular) and contractility.